Cytochrome P-450s are a superfamily of enzymes, some of which are capable of metabolizing xenobiotics such as drugs and carcinogens as well as endobiotics such as steroids and prostaglandins. In animals, multiple forms of these enzymes are expressed simultaneously either constitutively or after administration of specific inducers. These enzymes display overlapping substrate specificities. Thus, a single cytochrome P- 450 may metabolize multiple substrates and a single substrate may be acted upon by several cytochrome P-450s. Some of the cytochrome P-450 catalytic products bind to cellular macromolecules and thus are presumed to initiate mutagenesis and carcinogenesis. In order to define the contribution of a given cytochrome P-450 to the metabolism of specific drugs and carcinogens, it is important to express these enzymes individually. For this purpose, we have begun to develop expression systems in which an individual cytochrome P-450 protein is synthesized from its full length cDNA. Success in this effort will enable us to define the contribution of each of these enzymes to mutagenesis and cell transformation mediated by chemical carcinogens. For this purpose, we employed two types of expression systems, namely recombinant vaccinia virus and recombinant retrovirus. We have constructed infectious recombinant vaccinia virus and infectious recombinant retrovirus containing the full length coding cDNA sequences for mouse cytochrome P1-450 and P3- 450. Human and mouse cells infected with the recombinant viruses expressed high levels of the authentic size proteins as detected by immunoblotting. The expressed proteins are enzymatically active and displayed substrate specificities characteristics of the respective enzymes. Experiments to determine the catalytic specificities and the contribution of the enzymes to mutagenesis are in progress.